With the advent of substanial new uses for electro/optic systems, there exists a greater need to effectively control the passage of electro-optic signals, both in their direction and time. This is especially true in phased array systems, although, it should be realized that this is just one of numerous electro-optic systems which require the use of an optical time shifting and routing mechanism.
Phased array systems are generally made up of arrays of many relatively isotropic radiators or emitters, spaced for example by half their radiating wavelength, that are each driven coherently but with a relative phase (or time) shift among individual elements or among subarrays of elements. Controlling this phase shift across the array of radiators permits the array to form a beam that is strongly peaked in the far-field. Using this well established technique, the direction of the beam can be steered electronically (much faster than is possible mechanically) by controlling the phase shifts. Further, the angular width of the beam decreases as the lateral extent of the array increases--driving the need for extensive arrays (e.g., in excess of one hundred wavelengths lateral extent) and thus for large numbers of elements. Even with subarraying techniques in which subsets of radiators are ganged to a common shifter so as to reduce the number of phase shifters required, the requirement for rapidly introducing phase or time delays into many parallel microwave channels forms a major technological challenge. It would therefore be highly desirable to provide digital time or phase shifting of signals for each emitter in a fast, accurate, compact, lightweight, inexpensive system while introducing minimal insertion losses and negligible spurious noise signals from scatter, reflections, and imperfect switch purity.
There are many practical barriers to implementing time delay networks directly in the microwave bands. These include difficulties such as: the power splitters used in such networks are large; the cables or waveguides used at microwave frequencies are bulky; the networks tend to be lossy; and dispersion of these delay lines makes the use of multiple bands difficult. Photonic technologies can be applied to this problem, for example, by converting the microwave signals to modulation on optical carriers, introducing the required delays optically, and then converting back into the microwave regime. This type of translation scheme permits the use of optical devices and techniques in phase shifting that are superior to those operating directly in the microwave regime.
For example, optical time delay networks can potentially be light weight, compact, and insensitive to electromagnetic crosstalk and interference. They can provide very long delays when required. Further, the dispersion effects are greatly reduced and multiple microwave bands can use the same delay network. Still, the advantages of using optical phase or time shifting must outweigh the overhead associated with converting to and from the optical regime.
Another technological challenge associated with driving phased array antennas arises in systems utilizing large bandwidth signals. When beam forming is accomplished by introducing phase delays (rather than time delays), large bandwidths cause the direction of the beam to detune from its desired direction. Since very large bandwidths are required for communication and identification of targets and tracking (as opposed to many searching tasks), true time delay beam forming networks are also important in high performance phased array systems.
In addition to the requirement for true time delay in beam forming for future phased array radar systems that utilize large bandwidth signals, it has been shown that time delay networks are needed with 1) low insertion loss (to reduce amplifier gain and resulting nonlinearities with strong signals); 2) low crosstalk among delay channels (to reduce amplitude and phase distortions in the resulting signal); and 3) low spurious signal generation (to reduce the formation of unwanted lobes in the array pattern). These additional requirements augment the technological challenge in future beam forming networks.
Recently there has been much attention to the application of photonic time delay networks for addressing the phased array beam forming problem. There are many openings for the application of optical approaches in radar systems, and the concept of optical beam forming with time delay networks has been physically demonstrated in W. Ng, A. A. Walston, G. L. Tangonan, J. J. Lee, I. L. Newberg, and N. Bernstein, "The First Demonstration of an Optically Steered Microwave Phased Array Antenna Using True-Time-Delay," Journal of Lightwave Technology, 9, 1124 (1991). The bulk of the optical approaches, however, have been directed at switching and delaying the optical carrier using guided waves such as in optical fibers or planar waveguides. Many of these techniques use combinations of integrated optical switches and guided wave delay lines.
Other approaches for optically introducing time or phase shifts which have difficiencies associated therewith include the use of heterodyning and coherent techniques and the use of segmented mirror spatial light modulators and polarization routing through prisms in free space.
It is quite apparent there is still much room for advancement in these prior approaches, particularly with respect to losses, complexity, crosstalk, switch isolation, compactness and multiple reflection suppression.
It is therefore an object of this invention to provide an optical time shifter and routing system which incorporates therein a free-space switching technique.
It is a further object of this invention provide an optical time shifter and routing system which has superior switch isolation, multiple reflection and crosstalk suppression; less complexity and lower insertion loss; and less stringent wavelength tolerances than time shifting systems of the past.
It is another object of this invention provide an optical time shifter and routing system which is extremely compact.
It is still another object of this invention provide an optical time shifter and routing system which utilizes a series of switchable gratings therein.
It is still a further object of this invention provide an optical time shifter and routing system which incorporates saturable absorber noise suppressors therein.